As discussed in the co-pending application, DC to AC power inverters employing switching elements are well known in the art. In one typical configuration, four bi-polar switching transistors of like conductivity type are arranged as a full wave bridge inverter. Transistors in first and second switch arms of the bridge are connected in a first series circuit between a DC power source and a bridge diagonal that is connected to a load so that current flows in a first direction through the diagonal during the first time interval, while the transistors in the first and second arms are both forward biased. Third and fourth switch arms of the bridge include transistors that are connected in a second series circuit between the DC source and the diagonal so that current flows in a second direction through the diagonal during a second time interval while the transistors of the third and fourth arms are forward biased. Forward biasing of the transistors of the first and second arms occurs during a first half cycle of a time reference AC source, while the transistors of the third and fourth arms are forward biased during alternate, second half cycles of the time reference source. Typically, forward biasing is provided in an out-of-phase relationship by the AC source being coupled between the base and emitters of the transistors of the first and second arms, and of the third and fourth arms.
Because the switching transistors of each pair of arms are essentially in series while they are forward biased into conduction, it is advantageous for the switching transition times of these transistors to be the same. It is particularly advantageous for the cut-off times of transistors in each pair of arms to be the same. The necessity for simultaneous transition times of the switching transistors of each pair of arms can be obviated if the transistors have very high power ratings. However, it is not advantageous to provide transistors with excessively high power ratings because of cost, heat dissipation and size.
If transistors with low power ratings are employed, they should have simultaneous conduction and cut-off times for numerous reasons. In particular, if low power transistors are simultaneously activated into the forward biased condition, turn-on losses of both transistors of one pair of arms are equalized to provide equal heat losses in the transistors of the first and second arms and symmetrical heat sinking. Simultaneous turn-off times of the transistors of a pair of arms distribute turn-off losses equally between the transistors. If the transistor of one arm turns off appreciably before the transistor of another arm, there is unequal power dissipation. If the transistor in one arm turns off first, that arm must sustain the full turn off switching loss. The transistor in the arm which is slower to turn off has essentially no dissipation during turn-off because the current through it is interrupted by the turn-off of the faster transistor. Equalizing the turn-on and turn-off times of the transistors of a pair of arms greatly reduces switching noise and transients. If the transistors of two series arms are simultaneously turned on and turned off, the inductive open circuit emitter-collector voltage of each transistor can be materially reduced; the inductive voltage being due to a collapsing magnetic field of the transformer in the diagonal.
It is difficult to obtain inexpensive, off-the-shelf transistors that are matched to have characteristics so that they are simultaneously activated into a conducting state and deactivated into a cut-off state. Of course, matched transistors, i.e., those having desired characteristics, exist, but the price of matched transistors is significantly greater than the price of unmatched, or off-the-shelf transistors. If high current capability is required, as can be achieved by employing parallel transistors, the difficulties in achieving matching between many transistors is appreciably greater than matching characteristics of only a pair of transistors.
In the co-pending application, the turn-on and turn-off times of a pair of unmatched, low power transistors in the two series circuits of a full wave bridge inverter are forced to be substantially the same by transformer coupling the emitters of the transistors in each pair to each other. A first transformer includes first and second windings connected as series elements of the first series circuit. Terminals of the first and second windings of the first transformer are respectively connected to the emitters of the two transistors of the first series circuit. A second transformer includes third and fourth windings connected as series elements of the second series circuit. The third and fourth windings have terminals respectively connected to the emitters of the third and fourth transistors. To provide the simultaneous turn-on and turn-off times, the windings of the first and second transistors are arranged so that the emitter voltages of the first circuit have a tendency to vary in the same direction, and the emitter voltages of the transistors of the second circuit have a tendency to vary in the same direction.
By activating the transistors of each circuit simultaneously into conducting and cut-off conditions, there are equalizations of the turn-on and turn-off losses of both transistors in a particular series circuit. Further, the needs for matched transistors, transistors having high emitter-collector sustaining voltages, or transistors having high power ratings is obviated. By activating the transistors simultaneously into the cut-off and conducting states, noise and transients are materially reduced, which also helps to lower the current ratings of back biased diodes that shunt the emitter-collector paths of each of the transistors. The shunt diodes absorb inductive voltages which might otherwise exist between the emitter and collector of the transistors as they turn off.
While the prior art bridge power inverter described in my co-pending application functions admirably for many situations, there are other, high power situations wherein a single transistor cannot supply adequate current to a high power load. It is well known that the current capability of a single transistor can be increased by connecting several transistors in parallel, so that the emitter-collector paths thereof are in parallel, and the bases of all of the transistors are connected to be biased by a common source. The parallel transistor connection is advantageous because, in many cases, the price of two relatively low power transistors is less than that of a single high power device. However, there are numerous problems associated with connecting unmatched parallel switching transistors to each other. In particular, the different parallel transistors have a tendency to have: (1) differing load currents flowing between the emitter and collector electrodes thereof, and (2) unequal turn-on and turn-off times. The unequal load current amplitudes cause uneven power dissipation in the different parallel connected transistors, and frequently in the eventual destruction of the inverter, as dissipation becomes greater. To obviate the problem of uneven power dissipation, it has been common to employ emitter "ballast" resistors. However, the use of ballast resistors is inefficient, because of the power consumed by the resistors; further, the ballast resistors do not eliminate the turn-on and turn-off switching problems and may have a tendency to increase these problems. The unequal turn-on and turn-off times of the parallel, switching transistors cause excessive power dissipation for the transistor which has a tendency to turn on first and excessive power dissipation for the transistor which has a tendency to turn off last. The excessive power dissipation occurs for these transistors because they have to carry most of the current during the period that the other transistors are not fully conducting.
It is, accordingly, an object of the present invention to provide a new and improved circuit for switching current from a DC source to AC current which is to be supplied to a load by employing switching elements that are connected in parallel with each other and which are simultaneously activated into conducting and cut-off states.
A further object of the invention is to provide a new and improved DC to AC inverter employing switching transistors that are connected in parallel with each other, wherein the transistors are substantially simultaneously rendered into conducting and cut-off states, and the current flowing through each of the transistors, while conducting, is approximately the same.
A further object of the invention is to provide a new and improved DC to AC inverter employing a pair of simultaneously energized switches that are connected to opposite electrodes of a DC source and to opposite terminals of an AC load, wherein each switch includes a plurality of parallel, unmatched elements that are substantially simultaneously activated into conducting and cut-off states, and which have approximately equal currents flowing through them while conducting.
A further object of the invention is to provide a new and improved DC to AC power inverter bridge, wherein each arm of the bridge includes a plurality of parallel, unmatched switching elements, and wherein the elements of a first pair of arms of the bridge are simultaneously activated into conducting and cut-off states, and the elements of third and fourth arms of the bridge are simultaneously activated into conducting and cut-off states, with the current flowing through each of the elements, while in the conducting state, being approximately the same.